Many electronic devices, such as microcontrollers, central processing units (CPU), memory devices, and the like, require a defined supply voltage. A linear voltage regulator may be used to provide such a defined supply voltage from an input voltage that is higher than the desired supply voltage. A linear voltage regulator includes a pass device, such as a transistor, connected between a supply input for receiving the input voltage and an output for providing the defined supply voltage to a load. A control circuit controls the pass device such that the supply voltage corresponds to a desired voltage.
The control circuit for a linear regulator may be implemented using analog control circuitry, digital control circuitry, or a combination of both. When digital control circuit and methods are used, the linear voltage regulator may include circuitry configured to sample the output of the power supply and convert the sampled output to the digital domain. Once the sampled output is processed in the digital domain, a control signal is applied to the pass device.
Some of the common specifications of a voltage regulator include load transient response, dropout voltage, output voltage ripple, and power supply rejection ratio. The load transient response pertains to how fast the voltage regulator responds to a changed load condition. This load transient response may be limited, for example, by stability considerations within the regulation loop. In some cases, the quantized and sampled nature of the digital controller may reduce the phase margin of the power supply.
The dropout voltage pertains to how low the regulator input voltage can approach the controlled output voltage and still maintain operation, and the voltage ripple pertains to the amplitude of a voltage disturbance is seen at the output voltage, which may be periodic in some cases. Again, the sampled and quantized nature of a digital controller may cause some voltage ripped in a digitally controlled linear voltage regulator. Lastly, the power supply rejection ratio pertains to how well the linear voltage regulator rejects changes in its supply voltage.